Either an Ulbricht sphere photometer or a goniophotometer is used in the normal manner in order to measure the light flux of a light/radiation source.
The sphere photometer comprises a hollow sphere with an internal surface which is painted matt-white and produces multiply diffuse reflections of the light from a light source which is fitted in the interior, so that each area element of the internal surface is illuminated with equal intensity. The light intensity of an opalescent glass pane which is fitted in a viewing hole is then proportional to the total light flux. In practice, the use of a sphere photometer is relatively problematic, since adequate measurement accuracy is ensured only after complex calibration of the sphere photometer and with the environmental conditions remaining constant and with low air humidity.
Thus, in practice, goniophotometers are preferred. These are mechanical/optical measurement systems by means of which, depending on the measurement appliance head that is used, the directional dependency of variables can be determined in order to describe the optical radiation, for example of the luminance intensity distribution bodies. If the light/radiation source is located at the origin of a polar coordinate system (r, θ, Φ), then the values of the measurement variable are measured goniophotometrically, that is to say successively in the angle ranges 0≦θ≦180°, 0≦Φ≦360° for all directions. The integration of the luminance intensity distribution body over the complete spatial angle results in the technically and financially important total light flux.
In the already known types of compact goniophotometers, the required relative movement of a measurement appliance head with respect to the light/radiation source takes place along the envelope surface of a (fictional) sphere with a radius of up to 3 m, with the measurement direction always pointing towards the center of the sphere, at which the light/radiation source is arranged. This relative movement is achieved by rotating about two axes, which intersect at right angles, with the angles (θ, Φ) at a constant radius, in which case, in principle, three combinations of the rotary movements are possible:
a) The light/radiation source is rotated about two axes (vertical and horizontal) with the measurement appliance head stationary. However, it has been found that the rotation about a horizontal axis results in an unacceptably major change in the photometric characteristics of the light source.
b) The light/radiation source and the measurement appliance head are each rotated in their own right about a vertical axis (source) and are moved on a circular path (measurement appliance head) about a horizontal axis. However, the rotations of the light/radiation source must be carried out very slowly in order to avoid changes in the photometric characteristics.
c) The measurement appliance head is moved on a family of circular paths along the envelope surface of a sphere with the light/radiation source stationary. Any desired mounting position of the light/radiation source and its fixed position during the measurement are in this case made possible, for example, by a complex universally jointed goniophotometer, as is described at www.ptb.de/de/org/4/41/412/kalibrierung.htm. In this case, the light/radiation source is held in a predetermined mounting position via an outer frame, while the measurement appliance head (photometer) is guided via two inner frames, which are connected to one another in a universally-jointed manner. A universally-jointed goniophotometer such as this can be produced only with very major hardware complexity. A further disadvantage of the solutions described above is that the measurement time for determination of the light flux and of the luminance intensity distribution is relatively long and, for example, this may take 80 minutes.
A further disadvantage of the known solutions is that different measurement appliance heads must be used to measure different variables, for example the luminance intensity distribution, the three-dimensional color distribution and the luminance distribution, so that a number of measurements must be carried out successively in order to record the individual variables.
The German Physical/Technical Federal Administration (PTB) have developed a robot-assisted goniophotometer with three robot units, by means of which the measurement times can be considerably shortened in comparison to the solutions described initially.
The multiple axis robots each have a horizontal rotation axis and are mounted on walls of a measurement chamber via a high-strength holder. One robot holds the light/radiation source at the predetermined mounting position, while the two other robots, which are mutually opposite, are each fitted with at least one measurement head, which is aligned with respect to the center, that is to say with respect to the mounting position. The robots are then driven appropriately in order to move the measurement heads with radii of up to 3 m around the light/radiation source along virtually any desired movement paths.
A goniophotometer such as this is used, for example, to provide the light flux unit and, owing to its complex structure and the stringent software and hardware requirements, is very expensive, and is thus suitable only for research into fundamentals and for use at the PTB.